FileMaker Go also uses the iOS SDK within FileMaker Go to access the camera. I suspect you have a companion application to access the external camera, and FileMaker Go would be unable to access that information. You would have to download the photo to the camera roll in order to have access via FileMaker Go.
Well this may possibly be able to open a second app that takes a photo and then imports it into FMGo. Very similar to what CNS barcode does with Barcode data just with photos. The issue is getting a photo back in with a reference to the camera roll so this is smooth for the users. I am not familiar with it but I will find someone who is. This may be a way for FMGo to finally get square images into square containers as external camera shoots square photos.